In recent years, organic electro-luminescence (organic EL) devices are drawing attention as one kind of light-emitting device with a high emission efficiency. FIG. 1 schematically shows a cross-sectional structure of a generic organic EL device. The conventional organic EL device is structured so that an electrode 2, an emission layer 3, a transparent electrode 4, and a transparent substrate 5 are stacked on a substrate 1. The transparent substrate 5 is in contact with an air layer 6. By applying a voltage between the electrode 2 and the transparent electrode 4, the illuminant emits light at a point S in the interior of the emission layer 3. Within the generated light, a portion intactly propagates along the direction of the air layer 6, while a portion is reflected at the electrode 2 and then propagates along the direction of the air layer 6. When light propagates from a medium with a high-refractive index to a medium with a low-refractive index, total reflection occurs if the incident angle of the light exceeds a critical angle. As a result, only the light which has propagated without undergoing total reflection before reaching the air layer 6 goes out to the exterior of the organic EL device. Assuming refractive indices nk, n0 of the emission layer 3 and the air layer 6, respectively, the critical angle θc is expressed by eq. (1).θc=sin−1(n0/nk)  (1)
Therefore, in the organic EL device shown in FIG. 1, the light which can be extracted out of the device is limited to the light which is emitted at the point S and strikes the interface between the air layer 6 and the transparent substrate 5 at an angle smaller than the critical angle θc. Assuming that the light emission from the point S is isotropic, and that the transmittance at the refraction plane is 100% at any incident angle equal to or smaller than the critical angle, with the effect of interference of light being ignored, then the rate by which light can be extracted (light extraction efficiency) η is expressed as 1-cos θc. For example, when the emission layer 3 has a refractive index of 1.7, the extraction efficiency η is less than 20%. Thus, generally speaking, an organic EL device does not have a high efficiency of light utility.
In order to increase the light extraction efficiency, a technique is being studied which adjusts the thicknesses of the emission layer 3 and the transparent electrode 4 on the order of light wavelengths so that light is efficiently extracted by utilizing the effect of interference of light. However, since the effect of interference depends on the direction in which generated light propagates, light in every direction cannot be efficiently extracted. Therefore, this method cannot attain a 100% extraction efficiency. Moreover, since the interference of light depends on wavelength, it is difficult to efficiently extract light of all wavelengths within the emission wavelength region. In the case of a white-color organic EL device, this causes luminance unevenness or color unevenness, where luminance or color changes depending on the angle of viewing.
An example technique of more efficiency extracting light from an organic EL device as such is disclosed in Patent Document 1.
Patent Document 1 discloses an organic EL device having an improved light extraction efficiency, where a diffraction grating is formed on the substrate interface or the reflection surface in order to change the incident angle of light at or beyond which total reflection will occur, relative to the interface.